1. Field of the Invention
The present invention relates to cured composites built from layers of unidirectional fibers. In particular, the invention utilizes highly porous lightweight materials in conjunction with multilayer preforms to obtain cured articles with improved toughness.
2. Description of Related Art
High-performance composite materials built of alternating layers of unidirectional reinforcing fibers have an advantageous combination of high strength and light weight. As such they find use in aerospace and other industries where such properties are critical. Generally, the composite materials are prepared by laying up a number of alternating layers wherein adjacent layers have unidirectional fibers running at different angles. The net effect of buildup of several layers of such unidirectional fabrics is to provide a composite material having exceptional strength, either quasi-isotropically, or in one or more particular directions.
Such composite materials may be produced as prepregs or as preforms. In prepregs, layers of unidirectional fabrics immersed or impregnated with a matrix material such as a resin are laid-up into the shape of the part to be produced from the composite material. Thereafter, the laid-up part is heated to cure the matrix material and provide the finished composite part. In the preform approach, layers of unidirectional reinforcing fibers or woven, braided, or warp-knit fabric are laid up similarly to the way they are laid-up in the prepreg method. However, in the preform method, the layers are laid-up dry, i.e., without the matrix material. Thereafter, the laid-up material is infused with the matrix material in a liquid-molding process, and the molded part is heated to cure the matrix material as in the prepreg method.
The alternating layers, or lamina, of reinforcing fibers provide the composite articles made from the prepreg or preform process with a great deal of strength, especially in directions that align with specific fiber directions. Accordingly, very strong lightweight parts may be produced, for example, as wings and fuselages of aircraft. Although the alternating lamina of reinforcing fibers provide strength, toughness or impact resistance is determined mainly by the properties of the cured matrix material. Impact-resistant or toughened matrix materials are generally preferred because they are resistant to damage from impact. This is important, for example, in the case of airplane wings made from such composite materials to avoid failure from foreign-object impact during flight, damage resulting from ground-maintenance impact (e.g. from tool drop, forklifts, or other vehicles), and the like. Furthermore, because impact damage in composite materials is generally not visible to the naked eye, it is important for such primary load-bearing structures to be able to carry their full design load after impact and prior to detection using non-destructive techniques.
In prepregs, the matrix material, which is typically an epoxy-based resin formulation, may be toughened by adding particles of a thermoplastic material to the conventional resin. These thermoplastic particles may either be soluble in the matrix resin and dissolve in the epoxy resin or may be insoluble and placed, during the prepregging operation (see, for example, U.S. Pat. No. 5,028,478) on the surface of each layer. Upon cure, the thermoplastic resin in the cured epoxy matrix serves to limit crack propagation through the part. Preform materials may also be stitched before resin infusion and cure to provide toughness and crack resistance. However, one drawback to stitching is the reduction of in-plane mechanical properties, particularly as the stitch density increases. The prepreg approach of applying particles of thermoplastic material in the resin before cure is not directly transferable to the liquid molding processes used to prepare preform articles. In the resin infusion of the liquid molding process, soluble thermoplastics tend to increase the melt-flow viscosity of the matrix resin unacceptably, while insoluble thermoplastic toughening particles tend to be filtered by the preform and thus will not be located uniformly between the plies in the preform.
In the European Patent EP 1 175 998 to Mitsubishi, laminated products formed of reinforcing fibers are provided in which thermoplastic resin layers are provided between layers of the reinforcement fiber. The thermoplastic resin layer is described in the form of a porous film, fiber, network structure, knitted loop, and the like. The laminated product uses a thermoplastic layer of sufficient permeability between the layers of reinforcing fibers so as not to inhibit liquid resin flow during a liquid molding process. One drawback inherent in processes such as those described in EP 1 175 998 is that the preform made of alternating layers of reinforcing fibers and thermoplastic resin layers are less than perfectly stable during resin infusion. As a result, the reinforcing fibers and the thermoplastic resin layer tend to move or shift during the liquid molding process. Such moving or shifting can be mitigated by stitching together the layers before infusion with the resin. Another drawback to the processes described in EP 1 175 998 is that they are primarily effective for hand lay-up operations and not for automated lay-up operations that would be more relevant in the fabrication of large aircraft parts or in the continuous production of broad goods.
It would be desirable to provide a molded article made by a preform process in which the reinforcing fibers are held tightly in relative orientation to one another. It would further be desirable to provide a process for making such a preform article in widths and lengths feasible for producing large-scale parts, such as airplane wings, from them.